Project Summary. Since the first descriptions of mating-type switching in budding yeast approximately 40 years ago, characterization of this process has led to numerous key advances in understanding the mechanisms of gene silencing (heterochromatin), cell-fate determination (mating-type), and homologous recombination. For example, the highly conserved NAD+-dependent protein deacetylase, Sir2, and other ?silent information regulator (SIR) proteins, were initially identified due to their roles in silencing the heterochromatic HML? and HMRa loci, which are maintained as silenced copies of the active MAT? and MATa loci, respectively. Mating-type switching occurs when a programmed dsDNA break is generated at the MAT locus by an endonuclease called HO. The break is then repaired through homologous recombination using either HML? or HMRa as a template, with a strong preference for switching to the opposite mating type. Such ?donor preference? in switching is directed by a cis-acting sequence adjacent to HML? called the recombination enhancer (RE). Numerous chromatin factors have been implicated in mating-type switching, but mechanisms involving chromosome structural dynamics have remained elusive. In this proposal we investigate a newly discovered process of coordinating the ?sensing? of an HO-induced dsDNA break at the MAT locus with requisite chromosome III structural changes. Within the RE we identified a strong binding site for Sir2, condensin, and cohibin (Lrs4/Csm1) at the promoter of a long non-coding RNA (lncRNA) gene called RDT1. This site maintains chromosome III in a switching- competent structural conformation. Sir2 normally represses RDT1 transcription in non-switching cells, but is redistributed to the HO-induced break site at MAT, which activates RDT1 transcription and triggers mating-type switching. We propose 3 specific aims that will determine not only how RDT1 lncRNA induces switching, but also investigate the roles of condensin and cohibin in the programmed structural reorganization of chromosome III. This provides a unique and well-defined system for elucidating condensin function outside of mitotic chromosome compaction, something currently missing in the field.